metrics doc page should mention how to add custom metrics

[trentm]

https://www.elastic.co/guide/en/apm/agent/nodejs/current/metrics.html currently doesn't mention custom metrics at all. One has to know there is the apm.registerMetrics(...) API to go find it at https://www.elastic.co/guide/en/apm/agent/nodejs/current/agent-api.html#apm-register-custom-metrics

The metrics doc page should have a section on custom metrics.